The present invention relates generally to gas turbine engines, and, more specifically, to a turbine nozzle therein.
In a gas turbine engine, air is compressed in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream therefrom through one or more turbine stages each including a turbine nozzle and rotor blades. Each turbine nozzle includes a plurality of circumferentially spaced apart hollow airfoil vanes extending radially between and joined to radially outer and inner bands. During operation, the hot combustion gases flow between the adjacent vanes to the turbine blades for extracting energy therefrom. Each passage between adjacent vanes has a preferred minimum throat area which must be accurately controlled for controlling efficiency of operation of the turbine. The turbine nozzle is typically cooled with a portion of air bled from the compressor for ensuring an effective useful life thereof.
Since the engine operates at varying output power, the turbine nozzle experiences increasing and decreasing temperature which varies due to the effects of the heating combustion gases and the cooling air. Experience has shown that the use of annular outer and inner bands for supporting the vanes is undesirable because they restrain thermal growth and contraction of the vanes causing unacceptably high transient thermal stresses which significantly reduce the useful life of the nozzle. The temperature and stress operation of the nozzle is relatively complex and also includes differential temperature gradients of the components which also create undesirable thermal stress during operation, with the annular outer and inner bands causing increased thermal stress and distortion.
Accordingly, modern turbine nozzles are formed in circumferential segments with one or more vanes being fixedly joined in a corresponding segment with arcuate outer and inner band segments, By segmenting the outer and inner bands, hoop restraint is eliminated and therefore the vane segments are allowed to thermally expand and contract without restraint. The resulting thermal stresses are significantly reduced for ensuring a suitable useful life of the nozzle. However, segmenting the turbine nozzle increases the complexity of the nozzle and the manufacturing process for ensuring efficient aerodynamic performance during operation. Each nozzle segment must be accurately constructed and machined individually, and then accurately assembled and aligned with the other nozzle segments to form the complete annulus. Additional care must be used for accurately creating the individual vane throat areas in each nozzle segment and between adjacent ones of the nozzle segments.
Furthermore, the segmented turbine nozzle creates potential leakage flowpaths between the segments which is undesirable if cooling air is allowed to leak into the combustion gas flowpath which decreases efficiency of the engine. Accordingly, the circumferential end faces of each of the outer and inner band segments is typically provided with a spline strip seal trapped within a pair of complementary slots for sealing the circumferential gaps between adjacent bands. The complex temperature environment of the turbine nozzle includes axial and radial temperature gradients which may cause misalignment of the spline seals during operation leading to undesirable leakage therearound.
Yet further, in low solidity turbine nozzles having relatively few vanes, the vane chords are relatively large, with a correspondingly large aspect ratio of the chord length over the chord height which increases the axial length of the end gaps which typically extend diagonally between the angled nozzle vanes. The longer joints are more prone to leakage. And, the long aspect ratio increases the potential mismatch of the spline seal slots during operation.
Another consideration is engine size. The smaller the engine, the more difficult it is to effect suitable spline seals. And, in single stage turbines which operate at high pressure ratio, cooling air leakage in the nozzle segments increases aerodynamic efficiency losses during operation.